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shut down the APU and inhibit start.
4-12 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
In the essential (flight) mode, failure of both probes will not cause a shut down.
Instead the FADEC reverts to a preset temperature signal (260ºC) to allow pneumatic
loading and normal operation of electrical power. Because of the loss of EGT control,
the APU should be shutdown unless required for another contingency (refer to QRH).
HOURMETER/CYCLE COUNTER
An hourmeter/cycle counter is powered on at 95% rpm and records the amount of
running hours and start cycles of the APU. It is deactivated when APU shutdown is
initiated.
FADEC COMPONENT CONTROL SCHEMATIC
CONTROL SYSTEM
The APU electrical control system consists of two major sections: FADEC and
electrical accessories. The FADEC is designed to execute precise control of the APU.
Programming within the FADEC controls the APU through all modes and operating
conditions. The FADEC monitors EGT, rpm, oil temperature and pressure, and
provides output signals for information on EICAS. The electrical accessories are used,
in conjunction with the FADEC, to perform the sensing and control functions required
to safely and reliably start and monitor the APU.
GX_04_006
Fuel
Control
Unit
Flow Divider
GEARBOX
Speed
Sensor
INLET
LCV
Load
Control
Valve
Surge
Control
Valve
SCV
PRIMARY FUEL
MANIFOLD SECONDARY FUEL
MANIFOLD
IGNITION
APU Bleed Switch
Thermo-
Couple
FADEC
(ECU)
L ENG BLEED R ENG BLEED
APU BLEED
AUTO
CLSD OPEN
XBLEED
AUTO
OFF ON
AUTO
OFF ON
AUTO
OFF ON
APU
RUN
OFF START
For Training Purposes Only
Sept 04
4-13
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
PNEUMATIC
The pneumatic bleed load system consists of the Load Control Valve (LCV) connected
to the bleed ducting and normally modulated by the FADEC. The LCV can either be
controlled automatically by the bleed management control system (AUTO selection),
or manually by the APU BLEED switch (ON selection) on the BLEED/AIR COND/
ANTI-ICE panel.
The LCV will not open when manually selected ON if:
• Anti-ice is active
• The left engine pressure regulating valve is manually opened
• The right engine pressure regulating valve and the crossbleed valve are manually
opened
If the left and right PRV and crossbleed valve are in to AUTO position, a manual ON
selection of the APU LCV will result in the BMC reconfiguring all the values to
accommodate pilot selection.
A Surge Control Valve (SCV) works as a “waste gate” to protect the APU compressor
from a potential to Surge (stall). The solenoid valve assembly (which allows
pneumatic air to open the SCV) is energized only when:
• The APU generator is providing electrical power and
• The APU LCV is CLOSED (i.e APU not supplying any bleed air to the ECS) and
• Atmosphere pressure of 7 psia is sensed (at and above 16,375’ during ISA
conditions)
The exhaust of the APU SCV is located on the left side of the fuselage, aft of the left
engine pylon. The operation of the SCV is transparent to the pilot.
4-14 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
APU CONTROL PANEL
START SEQUENCE
The APU requires 28 VDC to start. APU start is automatic following switch selections
of RUN and START on the APU control panel.
The start sequence is initiated by selecting the APU control switch to RUN:
• APU IN BITE (on ground only) advisory message comes on
• APU rpm and EGT indications appear on the EICAS STAT page
• APU inlet door opens (on ground)
• APU fuel SOV is commanded open and right DC auxiliary fuel pump is energized
• APU IN BITE (on ground) advisory message goes out (approximately 10
seconds)
Select the APU control switch to START position (spring-loaded from START to
RUN) for greater than one second and note the following:
• In flight, the APU IN BITE message would now annunciate momentarily
• At approximately 5 percent rpm, fuel and ignition occur
• Between 46 and 60 percent the starter disengages, and
• RPM accelerates to 100 percent
• For ground starts, ignition is deactivated at 50 percent. In flight, ignition is
deactivated at 98 percent
APU
RUN
OFF START
RUN
• Initiates the APU prestart BIT.
• On ground, commands the inlet door to full open.
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